Gene expression of somatostatin receptor subtypes SSTR2a, SSTR3 and SSTR5 in peripheral blood of neuroendocrine lung cancer affected patients

Abstract

Somatostatin (SS) acts as a universal endocrine off-switch, and also inhibits the growth of neuroendocrine tumours through its specific receptors (SSTRs). Somatostatin receptors are G-protein-coupled receptors, which are encoded by five separate genes (SSTR1-5). Short peptide analogues demonstrate specific binding only for the subgroup consisting of SSTR2a, SSTR3 and SSTR5. Moreover, previous studies reported that expression of mRNA for SSTR2a correlated with therapeutic outcome in patients with carcinoid tumours treated with somatostatin analogs. To develop and apply a Real Time Quantitative PCR technique (RT-qPCR) to compare and contrast the mRNA levels of SSTR2a, SSTR3 and SSTR5 in Neuroendocrine Lung Cancer affected patients. Peripheral blood samples from 21 neuroendocrine lung cancer affected patients (14 SCLC, 6 LC and 1 LCNEC) subjected to scintigraphy with (111)In-DTPA-D-Phe(1)-octreotide (OctreoScan) and 24 healthy blood donors were investigated by RT-qPCR. mRNA levels for SSTR2a, SSTR3 and SSTR5 were measured in peripheral blood samples with a relative quantification method using plasmid dilutions as calibration curves and GAPDH as reference gene. A statistically significant increase in target genes/GAPDH copy number ratio was found for SSTR2a (median 38; IQR 22-141) and SSTR5 (median 51; IQR 19-499) in neuroendocrine lung cancer affected patients as compared with samples from healthy blood donors (P ≤ 0.0003 and P ≤ 0.0005). Since low levels of expression were detected in the control group for all three genes, optimal cut-off values were assessed using ROC curve analyses and were equal to 9.05 for SSTR2a and 16.97 for SSTR5. These cut off values resulted in a sensitivity of 86% (95%IC 65-95) for both markers and a specificity of 83% (95%IC 64-93%) and 79% (95%IC 60-91%) for SSTR2a and SSTR5 respectively. Comparison between OctreoScan results and RT-qPCR analysis demonstrated agreement in 76% of the cases. Our results suggest that SSTR2a and SSTR5 mRNAs are detectable in peripheral blood of neuroendocrine lung cancer affected patients using real-time quantitative PCR, with a good agreement with OctreoScan. The high sensitivity of this non-invasive molecular technique suggests that this method could represent a useful tool in the clinical management of neuroendocrine lung cancers.